Connecting Analog Voltage Signals to a DAQ Device

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Before You Begin

This document provides step-by-step instructions for wiring and configuring your DAQ device for analog voltage measurements. Before you begin using your DAQ hardware, you must install your application development environment and NI-DAQmx driver software. Refer to the Installing LabVIEW and NI-DAQmx document for more information. 

Analog Measurement Fundamentals

To measure analog signals, you need to know the signal source—grounded or floating. You also must consider the measurement system—differential, referenced single-ended (RSE), or nonreferenced single-ended (NRSE).

  • Floating Signal Sources — In a floating source, the voltage signal is not connected to any absolute reference or common ground, such as earth or building ground. Floating signal sources are also called nonreferenced signal sources. Some common examples of floating signal sources are batteries, thermocouples, transformers, and isolation amplifiers.
  • Grounded Signal Sources — A grounded source is one in which the voltage signals are referenced to a system ground, such as earth or building ground. Because such sources use the system ground, they share a common ground with the measurement device. The most common examples of grounded sources are devices that plug into the building ground through wall outlets, such as signal generators and power supplies.
  • Differential Mode — A differential measurement system has neither of its inputs tied to a fixed reference, such as earth or building ground. A differential measurement system is similar to a floating signal source in that the measurement is made with respect to a floating ground that is different from the measurement system ground. Differential connections may require two analog input terminals to make a signal connection, reducing the overall number of signals you can attach to your DAQ device.
  • Referenced Single-Ended Mode — An RSE measurement system measures voltage with respect to the ground, AIGND in the figure, which is directly connected to the measurement system ground. This option typically allows for more signal connections per device (up to 2X) than differential mode.
  • Non referenced Single-Ended Mode — In an NRSE measurement system, all measurements are still made with respect to a single-node analog input sense (AISENSE), but the potential at this node can vary with respect to the measurement system ground. This option typically allows for more signal connections per device (up to 2X) than differential mode.
  • Pseudodifferential Mode — Like a differential input channel, a pseudodifferential measurement system exposes both the positive and negative sides of the channel. You connect the positive and negative inputs to the respective outputs of the unit under test. The negative input is tied to system ground through a relatively small impedance The positive and negative sides of the input channel are separated by a larger impedance.

The following table provides an application-independent summary of analog input connections.

Table 1. Analog Input Configurations

Connecting Floating Signal Sources

A floating signal source is not connected to the building ground system, but has an isolated ground-reference point.

When to Use Differential Connections with Floating Signal Sources

Use differential input connections for any channel that meets any of the following conditions:

  • The input signal is low level (less than 1 V).
  • The leads connecting the signal to the device are greater than 3 m (10 ft).
  • The input signal requires a separate ground-reference point or return signal.
  • The signal leads travel through noisy environments.
  • Two analog input channels, AI+ and AI–, are available for the signal.

Figure 1. Differential Connections for Floating Sources

Differential signal connections reduce noise pickup and increase common-mode noise rejection. Differential signal connections also allow input signals to float within the common-mode limits of the instrumentation amplifier on the DAQ device.

When to Use NRSE Connections with Floating Signal Source

Use NRSE input connections only if the input signal meets the following conditions:

  • The input signal is high-level (greater than 1 V).
  • The leads connecting the signal to the device are less than 3 m (10 ft).

Figure 2. NRSE Connections for Floating Signal Sources

Differential input connections are recommended for greater signal integrity for any input signal that does not meet the preceding conditions. However, differential input connections may reduce the overall number of signals you can attach to your DAQ device.

With this type of single-ended connection, the instrumentation amplifier rejects both the common-mode noise in the signal and the ground potential difference between the signal source and the device ground.

When to Use RSE Connections with Floating Signal Sources

Use RSE input connections only if the input signal meets the following conditions:

  • The input signal can share a common reference point, AI GND, with other signals that use RSE.
  • The input signal is high-level (greater than 1 V).
  • The leads connecting the signal to the device are less than 3 m (10 ft).

Figure 3. RSE Connections for Floating Signal Sources

Differential input connections are recommended for greater signal integrity for any input signal that does not meet the preceding conditions. However, differential input connections may reduce the overall number of signals you can attach to your DAQ device.

With this type of single-ended connection, the instrumentation amplifier rejects both the common-mode noise in the signal and the ground potential difference between the signal source and the device ground.

Connecting Ground-Referenced Signal Sources

A ground-referenced signal source is a signal source connected to the building system ground. The difference in ground potential between two instruments connected to the same building power system is typically between 1 mV and 100 mV, but the difference can be much higher if power distribution circuits are improperly connected. If a grounded signal source is incorrectly measured, this difference can appear as measurement error. Follow the connection instructions for grounded signal sources to eliminate this ground potential difference from the measured signal.

When to Use Differential Connections with Ground-Referenced Signal Sources

Use differential input connections for any channel that meets any of the following conditions:

  • The input signal is low level (less than 1 V).
  • The leads connecting the signal to the device are greater than 3 m (10 ft).
  • The input signal requires a separate ground-reference point or return signal.
  • The signal leads travel through noisy environments.
  • Two analog input channels, AI+ and AI–, are available.

Figure 4. Differential Connections for Ground-Referenced Signal Sources

Differential signal connections reduce noise pickup and increase common-mode noise rejection. Differential signal connections also allow input signals to float within the common-mode limits of the instrumentation amplifier on the DAQ device.

When to Use NRSE Connections with Ground-Referenced Signal Sources

Use NRSE input connections only if the input signal meets the following conditions:

  • The input signal is high-level (greater than 1 V).
  • The leads connecting the signal to the device are less than 3 m (10 ft).
  • The input signal can share a common reference point with other signals.

Figure 5. Single-Ended Connections for Ground-Referenced Signal Sources (NRSE Configuration)

Differential input connections are recommended for greater signal integrity for any input signal that does not meet the preceding conditions. However, differential input connections may reduce the overall number of signals you can attach to your DAQ device.

With this type of single-ended connection, the instrumentation amplifier rejects both the common-mode noise in the signal and the ground potential difference between the signal source and the device ground.

When to Use RSE Connections with Ground-Referenced Signal Sources

Do not use RSE connections with ground-referenced signal sources. Use NRSE or differential connections instead.

As shown in the bottom, rightmost cell of Table 1, there can be a potential difference between AI GND and the ground of the sensor. In RSE mode, this ground loop causes measurement errors.

Field Wiring Considerations

Environmental noise can seriously affect the measurement accuracy of the device if you do not take proper care when running signal wires between signal sources and the device.

In the single-ended modes, more electrostatic and magnetic noise couples into the signal connections than in differential configurations. The coupling is the result of differences in the signal path. Magnetic coupling is proportional to the area between the two signal conductors. Electrical coupling is a function of how much the electric field differs between the two conductors. With this type of connection, the instrumentation amplifier rejects both the common-mode noise in the signal and the ground potential difference between the signal source and the device ground.

The following recommendations apply mainly to when you route analog input signals to the device , although they also apply to signal routing in general. Minimize noise pickup and maximize measurement accuracy by taking the following precautions:

  • Use differential analog input connections to reject common-mode noise.
  • Use individually shielded, twisted-pair wires to connect AI signals to the device. With this type of wire, the signals attached to the positive and negative input channels are twisted together and then covered with a shield. You then connect this shield only at one point to the signal source ground. This kind of connection is required for signals traveling through areas with large magnetic fields or high electromagnetic interference.

Refer to the document, titled Field Wiring and Noise Considerations for Analog Signals for more information.

Locating Your DAQ Device Pinout

Before connecting any signals, locate your device pinout.

  1. Open Measurement & Automation Explorer (MAX) and expand Devices and Interfaces.
  2. Right-click on your device name, and then select "Device Pinouts."

Figure 6. Device Terminals Help

The following terminal types correspond with analog voltage measurements:

  1. AI X(+/-): Analog Input — This is usually followed by a number corresponding to a channel number. For instance, if your DAQ card has 16 analog input channels, your pinout diagram probably shows positive and negative terminals for AI 0 through AI 15 located throughout the connector. The + and – symbols refer to differential input channel pairs.
  2. AI GND: Analog Ground — This terminal is electrically connected to the ground of the instrumentation amplifier on your DAQ device.
  3. AI SENSE — The AI SENSE terminal is a common reference point for certain single-ended input terminal configurations (NRSE and RSE).

Configuring an Analog Voltage Measurement

You can use MAX to quickly verify the accuracy of your measurement system Set Up. Using an NI-DAQmx Global Virtual Channel you can configure a voltage measurement without any programming. A virtual channel is a concept of the NI-DAQmx driver architecture used to represent a collection of device property settings that can include a name, a physical channel, input terminal connections, the type of measurement or generation, and scaling information. Follow these steps to begin:

  1. With MAX open, right-click Data Neighborhood and select Create New.
  2. Select NI-DAQmx Global Virtual Channel and click Next.
  3. Select Acquire Signals » Analog Input » Voltage

Figure 7. Creating an NI-DAQmx Virtual Channel

  1. Select the physical channel that you will connect your voltage measurement to. A physical channel is a terminal or pin at which you can measure or generate an analog or digital signal. A single physical channel can include more than one terminal or pin, as in the case of a differential input channel.

Figure 8. Device Physical Channels

  1. Click Next and enter a name for the global virtual channel or leave the default.
  2. Click Finish and you should see the following screen in MAX:

Figure 9. Setting Up a Voltage Channel in MAX

  1. On the settings tab, type in the minimum and maximum voltage values you expect to read from your measurement (-10 V to 10 V by default). Doing this allows you to achieve more resolution in your measurement because the digital codes of the analog-to-digital converter are spread evenly across the analog input range. So, if you expect and input a smaller range, the voltage input difference between two consecutive codes is smaller.
  2. Select your Terminal Configuration (Differential, RSE, NRSE, or Pseudodifferential) according to your wiring and the information above.

Connecting an Analog Voltage Signal to Your Device

The next step is to physically connect the voltage signal to your DAQ device.

  1. Click the Connection Diagram tab in MAX to continue.

Figure 10. Analog Voltage Connection Diagram

The connection diagram above indicates which pins on your DAQ device should be wired according to the physical channel you selected. In this example, a differential input connection uses pins 68 and 34, corresponding to ai0 (AI+) and ai8 (AI-). The diagram shows an SCC-68 terminal block accessory. You can change this by right-clicking on the device name under Devices and Interfaces and selecting Properties.

Testing the Signal

With NI-DAQmx global virtual channels, you can preview your measurements.

  1. With MAX still open, click back on the NI-DAQmx Global Channel tab and click on the Run button. The voltage value of your measurement displayed at the top of the screen.

Figure 11. Previewing a Voltage Measurement in MAX

You can choose to view the signal in tabular form or as a graph by selecting Graph from the Display Type pull-down menu. You also have the option of saving your NI-DAQmx Global Virtual Channel should you want to refer to this configuration screen again in the future.

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